This invention generally relates to methods for reducing or eliminating camber (i.e., arching) in wound or coiled ribbon or tape made of plastic material. In particular, the invention relates to methods for reducing or eliminating camber in a fastener tape comprising mutually interlocked thermoplastic zipper strips of the type used in the automated production of reclosable packaging.
In accordance with one manner of producing reclosable packaging, a pair of mutually interlockable zipper strips are formed separate from and then attached to a web of bag making or package making material. The zipper strips are attached to the film web using automated equipment. The feeding of zipper strips from rolls or spools for application in whatever length desired to bag making or package making sheet material is well known in the art.
A major problem with rolled or spooled extruded plastic zipper strips is that they develop curvature and camber. Winding of any flexible plastic extrusion in whatever configuration inherently distorts straightness of the finished product, which is visible when the strips are removed from the spool or reel. This is almost an inevitable result of the winding systems employed which, irrespective of taper, tension or torque winding, require that sufficient tension be applied to the extruded product to ensure accurate placement of that product on the spool or reel. As a result, the extruded product is in a state of flux in that it has not achieved its final fully crystallized condition before being wound due to production speed requirements. Therefore, in the extruded wound product, malleable condition stresses and strains are set into the product. As the strips are wound, subsequent shrinkage takes place over a period of time. The shrinkage virtually ensures that irrespective of the type of winding or plastic material used, the strips will tighten down to an increased degree around the spool circumference. For example, if a length of extruded plastic zipper strip were laid out on a floor, it would shrink at a known rate provided that both ends of the product are not trapped. The same length of zipper strip, when wound onto a spool and trapped on either end, cannot shrink over a period of time. Therefore, the strip has locked-in stress and strain that cannot be relieved.
In the case of extruded plastic flanged zipper strips of the type used in the automated production of reclosable packaging, a pair of interlocked zipper strips are wound or coiled on a flanged spool with the zipper flanges disposed generally parallel to the surface of the spool cylinder. Such interlocked flanged zipper strips, when unwound or paid out from the spool, typically have some degree of camber. As used herein, the term “camber” refers to the arching of the interlocked flanged zipper strips in a plane generally parallel to the zipper flanges. In other words, even when the interlocked zipper strips are unwound and cut into zipper segments, the zipper segments (when placed on a planar surface with one flange in contact with that surface) will have varying degrees of arching in the horizontal plane (as seen in FIG. 2, described hereinafter).
The deviation from a straight line or warpage in the extruded plastic zipper strips that results from being wound on a spool can cause problems when the zipper strips are cut into segments and those zipper segments are attached to a web of package making material. In most cases, it is desirable that the zipper strips be substantially straight during the zipper-to-web joining operation. In accordance with known methods of applying zipper strips to a web of package making material (e.g., transverse application of zippers on a vertical form-fill-seal machine or zipper application on a horizontal pre-made bag machine), the zipper strips are kept under tension in order to minimize the difficulty in applying the otherwise cambered zipper strips to the web. In cases where zero deviation from straightness is required, the zippers trips must be held under extreme tension. In applications where the zipper strips cannot be held under tension, application of the zipper strips to the packaging material is virtually unattainable.
A further complication arises from the fact that in many cases, the amount of camber in the wound fastener ribbon or tape varies as a function of the axial position of the cambered portions on the spool. In those cases, it has been observed that the ribbon portions wound in the regions adjacent the spool flanges have more camber than the ribbon portions wound in a central region of the spool. This may be attributable in part to the fact that in the central region, the fastener ribbon is wound generally helically, whereas in the vicinity of the flanges the helical winding of the fastener ribbon reverses direction. If a rotating mechanism is used to drive the travel of the winding portion of the ribbon or tape back and forth across the spool, at each spool flanges the rotating mechanism must hesitate while it reverses direction. When the rotating mechanism hesitates, the tension in the wound ribbon or tape tends to increase, which also contributes to the creation of camber in those portions of the ribbon or tape wound in the vicinity of the spool flanges.
There is a need for a method of winding plastic ribbon on a spool in a manner that reduces the degree of ribbon camber near the spool flanges.